An engine for motor vehicles such as automobiles is operated through an extremely wide area of engine speed ranging froman idling speed to the maximum speed and within a widely varying load range, and the quantity of its exhaust gas varies by a large margin. In an exhaust gas turbine having a single flow characteristic, therefore, it is not possible to recover and utilize the energy of the exhaust gas discharged from an engine thoroughly. So, a variable-volume turbocharger device has been already proposed in which a partition wall is provided in a turbine housing to divide the exhaust gas passage in said housing into two exhaust gas passages different in the flow characteristics and valve means are provided which are switchable to open either one of said divided exhaust gas passages, wherein said valve means are switched over to operate in accordance with the operating conditions such as the engine speed and load, thereby to improve the operational efficiency of the exhaust gas turbine.
In such a conventional variable-volume turbocharger device, two turbine flow characteristics can be obtained, but it is desirable to obtain plural turbine flow characteristics suited for the operating conditions of an engine.
The turbine housing of a turbocharger device can not be expected to be highly precise because it is manufactured by casting. Thus, it is difficult to form said partition wall and a turbine rotor in close relationship with high precision and a large clearance must be provided between the partition wall and the turbine rotor. In this case, the exhaust gas passage leading from the fore end of the partition wall to the turbine rotor is rapidly enlarged, with a disadvantage of resulting in loss.